This invention relates to a device for filtering systems for the separation of minute magnetizable particles down to particle sizes below 1 .mu.m from a gaseous or liquid medium introduced into a working volume permeated by a magnetic field and to a method for the operation of this device.
Magnetic filtering systems make use of the fact that a magnetizable particle in a suitable magnetic field is subjected to a force which moves or retains it against other forces acting upon it, such as gravity or the hydrodynamic friction forces acting on it in a liquid medium. Separating methods according to this principle can be applied, for example, to steam or cooling water loops in both conventional and nuclear power plants. For in the liquid or gaseous medium of these loops minute particles which have generally developed through corrosion are suspended. These particles are ferromagnetic, such as magnetite (Fe.sub.3 O.sub.4), partly antiferromagnetic, such as hematite (.alpha.--Fe.sub.2 O.sub.3), or paramagnetic like copper oxide (CuO). Accordingly, these particles which, in addition, occur in various particle sizes, are magnetizable to different degrees.
Large and/or strongly magnetic, i.e. ferromagnetic, particles can be separated by magnetic ball filters, for instance. Filtering equipment suitable for this purpose is known from the U.S. Pat. No. 3,539,509 and contains a cylindrical filter tank filled with soft iron balls which are disposed in a constant magnetic field generated by an electric coil surrounding the filter tank. The field strength gradients obtained through this magnetic field, in conjunction with the balls, are high enough to cause the ferromagnetic particles transported by a liquid flowing through the filter to accumulate at the magnetic poles of the balls. To clean this filter, the balls can be demagnetized.
However, minute ferromagnetic particles of a diameter in the order of magnitude of 1 .mu.m, or also weakly magnetic, i.e. antiferromagnetic or paramagnetic particles, can hardly be separated by this known device because the magnetic field gradients brought about at the soft iron balls are insufficient therefor. Therefore, the separating rate of this filtering device is too poor for these types of particles. The separating rate is understood to be the difference 1-p, p being the permeability of the filter structure. This permeability is defined as the ratio of the concentration of suspended substances still present in the medium after passing through the filter structure to the corresponding concentration prior to entering the filter structure.
A filtering device for the separation of such minute ferromagnetic or also paramagnetic particles is known from the U.S. Pat. No. 3,567,026. This filtering device contains a filter structure of ferromagnetic, noncorroding steel wool, disposed in a constant, strong magnetic field, the magnetic flux density of which, in the filter volume, is at least 1.2 Tesla. This technique is known as the high-gradient magnetic separation technique. In order to obtain a relatively high separating rate with such a magnetic filter, the steel wool wires must be very thin, on the one hand. For, the magnetic field gradients then produced at their surfaces are correspondingly great. On the other hand, however, the flow channels formed between the wires must also be large enough to prevent their clogging with separated material and to prevent the filter's flow resistance and the pressure drop brought about thereby from becoming too great. But, a steel wool for this magnetic filter which will meet these requirements is relatively difficult to produce. Moreover, a relatively high separating rate is attainable with this known magnetic filter only if a correspondingly large filter volume is available. The magnet coils for the generation of the high magnetic fields required must be of correspondingly large size. Therefore, only superconducting magnets can generally be used in the known filtering device.